--> Case Study of a Tight Gas Reservoir Offshore South Africa: Its Significance in Exploration and Exploitation

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Case Study of a Tight Gas Reservoir Offshore South Africa: Its Significance in Exploration and Exploitation

Abstract

The recent development of an “unconventional” tight gas reservoir offshore South Africa has brought to the fore many challenges faced by an operator. The reservoir is highly compartmentalized, has low porosity and permeability under high temperature and pressure (HPHT) conditions, comprising of tidally-influenced shallow marine deposits varying from amalgamated sandstone to heterolithic facies. Average reservoir porosity and permeability is 9% and 1mD respectively. The presence of diagenetic secondary porosity caused by leaching and preserved by overpressure has led to the development of high poro-perm streaks, where porosity reaches up to 16% and permeability 100mD. The reservoir is deep, having been buried below 3600m. The reservoir is very competent with UCS values of 35000psi. The challenges associated with such reservoir properties make for harsh drilling conditions and complex development solutions. Numerous studies were done and solutions executed in an effort to better understand and combat the challenges faced while developing the field, beginning with the completion design. Encouraging tests from seven appraisal wells could not solely guarantee a sustainable gas rate over production time, inducing the execution of an array of completion strategies to overcome this challenge. Three production wells were drilled with a long horizontal section, over 1500 m in length, each with a different completion strategy; Well A: un-cemented and perforated, Well B: slotted liner, Well C: barefoot. All wells were drilled parallel to the maximum stress direction with the aim of maximizing wellbore stability. Even with the implementation of multiple completion methods, the challenges posed by the field continually required improved understanding to wholly interpret the drilling and production results. Numerous quantitative analysis were carried out to better characterize the reservoir, stratigraphy and structural compartmentalization as well as the enhanced poro-perm distribution. The choice of drilling mud was recommended based on an analogue field extracted from a database of tight gas reservoirs, and a real-time geosteering approach assisted in well placement. Even with all these measures in place, more is needed to fully realize the potential of such a field, opening the opportunity to unlock immense resources of similar character.